(a) Field of the Invention
The present invention relates to a non-aqueous electrolyte additive for improving safety and a lithium ion secondary battery comprising the same, and more particularly to a non-aqueous electrolyte additive that can improve cycle life and safety properties of a lithium ion secondary battery.
(b) Description of the Related Art
In concert with the development of the information electronics industry, studies regarding batteries, which are referred to as the heart of all electronic equipment, are actively in progress. Specifically, a lithium ion secondary battery with a light weight and a high capacity has attracted attention as one of the essential parts of electronic equipment with the miniaturization of high-technology electronics equipment.
The lithium ion secondary battery uses a transition metal complex oxide such as lithium, cobalt, etc. as a cathode active material; a crystallized carbon such as graphite as an anode active material; and an aprotic organic solvent in which lithium salts such as LiClO4, LiPF6, etc. are dissolved as an electrolyte. Such a battery has high performance and a light weight, rendering it suitable as a battery for miniature electronic equipment such as cellular phones, camcorders, notebook computers, etc. However, this battery has many safety problems since it does not use an aqueous electrolyte but instead uses a highly flammable organic solvent as an electrolyte. Safety is the most important aspect of a lithium ion secondary battery using a non-aqueous electrolyte, and specifically, preventing short circuits and overcharging problems are very important factors thereof.
In the lithium secondary battery, the method of stopping a fire of the battery due to a short circuit of the battery have not been well known except a design aspect of the battery. As the capacity of the battery increases, the discharge current increases, and therefore the dangerousness due to a short circuit becomes more serious.
Recently, in order to prevent overcharge, a various additive is developed. This is method that an organic materials added as an additive is decomposed on electrode surface to intercept a current by forming a polymer insulating film on the electrode when reaching fixed overvoltage. However, although the method has superior effect on overcharge, it has a problem reduced cycle life of the battery due to a necessity a many additive in order to reveal its effect.
In addition to, a method involves using electronic circuit shch as the method a mechanically intercepting a current by aggravating the occurrence of gas when overcharging, or the method shutting down a circuit by melting a separator.
As a further method, it is the use of a chemical reaction using a suitable redox shuttle additive in the electrolyte of the lithium battery. However, the method may be effective since overcharge current between a negative electrode and a positive electrode can be consumed, only on condition that reversibility of oxidation-reduction reaction is excellent.
In relation to this method, Japanese Patent Publication No. 1-206571 (1989) discloses that ferrocene compound is applied in the 3 Voltage class batteries, which can be a useful method in terms of cost efficiency. However, because the practical lithium secondary battery has the voltage of about 4V region, the battery needs oxidation-reduction agent un-decomposed in the voltage region.
Japanese Patent Publication No. 7-302614 (1995) proposes a new redox shuttle additive applicable in 4V grade, which combines the alkyl group (R: CnH2n+1) or the methoxy group (OCH3) as the electron-donating group of benzene ring. However, because the amperage capable of reversible redox shuttle is low, the effect has been defective in case of high amperage.
U.S. Pat. No. 5,709,968 (1998) discloses that the capability of a redox shuttle in the battery of 4V grade is improved largely by combining benzene with halogen. However, as amperage inflicted in case of overcharge increases with a high capacity of battery, a concentration of additive capable of consuming a current also increases as an adverse effect in a cycle life etc.